Move SignalProtocolStore off window

ts/types/Capabilities.d.ts

@@ -0,0 +1,12 @@
+// Copyright 2020 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+// When updating this make sure to update `observedCapabilities` type in
+// ts/types/Storage.d.ts
+export type CapabilitiesType = {
+  attachmentBackfill: boolean;
+};
+export type CapabilitiesUploadType = {
+  attachmentBackfill: true;
+  spqr: true;
+};